Method and device for adjusting a camera

ABSTRACT

For focussing and/or adjusting of the image field of a camera ( 1 ) with an image sensor ( 2 ) and an objective ( 3 ), it is proposed to provide light sources ( 7 ) between objective ( 3 ) and image sensor ( 2 ), which are projected into the object plane ( 5 ) by means of a partially transmitting mirror ( 8 ) arranged between the light sources ( 7 ) and the image sensor ( 2 ).

BACKGROUND OF THE INVENTION

This invention is related to a method and to a device for adjusting acamera with an image sensor for taking a picture and with an objectivefor imaging an object on the image sensor.

During the use of cameras, for example in the scope of an industrialproduction for the monitoring, the control and the regulation ofproduction procedures, it is necessary to adjust the camera. For this,the adjustment comprises the image sharpness setting of the opticalimaging by appropriate corrective measures, as e.g. the change of theimage width or object width of the optical imaging, or the adjustment ofthe objective, as well as the alignment of the position of the imagefield, which must correspond to the required visual field on the object,and which is adjusted by the position or by the alignment of the imagesensor. These adjustment measures must be performed on the cameradirectly and on site.

In case of camera systems with image sensors, a video signal or avideo-similar signal is often available for this purpose, allowing todisplay the image taken by the image sensor on a locally connected videomonitor, so that the sharpness and the position of the picture taken canbe controlled using the display of this monitor. This method, however,has the disadvantage that the camera must be equipped with asophisticated electronic system for generating the video signal requiredon site, and that the camera can only be adjusted if a connected monitoris available for this purpose.

Another known possibility to check and to adjust the sharpness and/orthe image field, even if no video signal or monitor is available, is toequip the camera with an autofocussing device which adjusts the correctsharpness automatically, making a manual adjustment of the sharpnesssuperfluous, as well as including an additional viewfinder in the cameraallowing to view the image field. This solution has the disadvantagethat an autofocussing device is technically sophisticated, as thedistance adjustment, in many application cases, does not have to beperformed often, e.g. only during the initial set-up, or in case ofmaintenance and repair measures. The viewfinder used for this has thedisadvantage that the image field cannot be recognized sufficiently inmany cases, in particular if the object width varies, and that thecorresponding space conditions do not always allow to view theviewfinder with the human eye.

For this reason, projection devices as to the state-of the-art are knownwhich project an adjustment mark onto the object plane with a projectiondevice; the sharpness of the image and/or the image field can becontrolled and adjusted with this adjustment mark. The adjustment marksare made visible by an appropriate screen, e.g. a simple piece of whitepaper.

A known projection system of this type uses two projection devicesindependent from the imaging system for the image sensor, each of themprojecting a circle onto the object plane. Both circles are identical indiameter and the projection is performed with an angle slightlydeviating from parallelism. The projection devices are adjusted in a waythat the two projected circles unite to form a single circle, in case ofa correct distance between the image sensor and the object to be taken,i.e. in case of optimum image sharpness. A third projection device isarranged, projecting a third circle, roughly indicating the size and theposition of the visual field, i.e. the range of the object displayed onthe image field of the image sensor.

Such known systems with projection devices are technicallysophisticated, as at least one further imaging optical system for theadjustment projection devices is necessary besides the underlyingimaging optical system of the camera. Furthermore, in case of a possiblereadjustment or change of the optical system, it is necessary to performa complete new adjustment of the projection device.

An optical system for which an automatic focussing process is performedis known from the document EP 0080340 B1. Here, the optical systemprojects light onto the object, the object reflects the light and theoptical system captures the light again; for focusing purposes, thelight is displayed on an image sensor which is arranged in the imageplane of the objective. Adjustment and setting on site using the humaneye is not possible here without using technical equipment forgenerating an image, captured by the image sensor for adjustmentpurposes. From document DE-OS 2413155 a comparable device is known whichuses two image sensors.

A projection device for adjusting the image field is known from documentU.S. Pat. No. 4,899,188, however, it does not solve the problem of thefocusing of a reproduction device.

An image field recognition device is known from the document GB 2257800A. For this, laser systems are used the light of which does not pass thecamera objective. Thus, a sharpness control in not possible.

SUMMARY OF THE INVENTION

Taking into account this state of the art, this invention has the objectto provide a method and a device for the adjustment of a camera which istechnically less sophisticated and which offers advantages in practicaluse.

According to the invention, this problem is solved by a device with thefeatures of the appended claim 1 and by a method with the features ofthe appended claims 17 and 18, respectively. Preferred embodiments andfurther developments of this invention can be taken from the dependentclaims, and the subsequent description with the corresponding drawings.

Thus, a camera according to the invention with an image sensor fortaking a picture and with an objective for imaging an object on theimage sensor, includes a device for adjusting the distance between thecamera and the object, or for adjusting the objective; this devicecomprises a partially transmitting mirror arranged between the objectiveand the image sensor, and at least one light source. Furthermore, itshows following special features: the partially transmitting mirror isarranged in parallel to the light-sensitive surface of the image sensor,and the at least one light source is arranged between the objective andthe partially transmitting mirror, emitting light in the directiontowards the partially transmitting mirror. For this, the distancebetween the light source and the mirror is as long as the distancebetween the mirror to the image sensor.

In a method according to the invention, for adjusting the distancebetween a camera and an object, or for adjusting the objective of acamera, wherein the camera comprises an image sensor for taking apicture and an objective for imaging an object onto the image sensor,the camera is equipped with a partially transmitting mirror arrangedbetween the objective and the image sensor, and at least one lightsource is provided. The partially transmitting mirror is arranged inparallel to the light-sensitive surface of the image sensor, the atleast one light source is arranged between the objective and thepartially transmitting mirror, emitting light towards the partiallytransmitting mirror. For this, the distance between the light source andthe mirror is as long as the distance between the mirror and the imagesensor, and the adjustment of the distance between camera and object, orthe adjustment of the objective, is performed according to the sharpnessof the image of the at least one light source in the object plane.

In a method according to the invention, for adjusting the position of acamera in relation to an object, wherein the camera comprises an imagesensor for taking a picture and an objective for imaging an object ontothe image sensor, the camera is equipped with a partially transmittingmirror arranged between the objective and the image sensor, and at leastone light source is provided. The partially transmitting mirror isarranged in parallel to the light-sensitive surface of the image sensor,and the at least one light source is arranged between the objective andthe partially transmitting mirror, emitting light towards the partiallytransmitting mirror. For this, the distance between the light source andthe mirror is as long as the distance between the mirror and the imagesensor, and the at least one light source is arranged in relation to theimage sensor in a way that its image is projected by the partiallytransmitting mirror and the objective to a place on the object, locatedin the border range of the visual field of the image sensor. Theposition of the camera in relation to the object is then adjustedaccording to the position of the image of the at least one light sourceprojected onto the object.

According to the invention, at least one light source is provided,serving as adjustment aid, wherein the image of the light source isprojected onto the object, serving as sharpness and/or image field mark.The sharpness and the image field, respectively, can be adjusted byviewing the image of the adjustment aid on the object. A very compactstructure is achieved by using a partially transmitting mirror, orientednormally to the optic axis; the adjustment expense is very low.

Many advantages stem from the fact that the imaging objective or theoptical path, respectively, for the imaging of the object onto the imagesensor are used for the original generation of the object image on theimage sensor, as well as for the projection of the at least one lightsource via the partially transmitting mirror into the object plane. Thedevice according to the invention is easy to build, technicallyinexpensive, requires only a small amount of adjustment and allows theuse of interchangeable objectives, which was hitherto impossible withrespect to the state-of-the-art. The adjustment can be performed easilyon site by viewing the light source projected onto the object withrespect to the sharpness of imaging or the adjustment of the image fieldwithout the necessity of additional, technically expensive equipment.

This invention achieves goals the branch has been looking for for a longtime already. In order-to achieve good results with respect to thepractical use as well as with respect to the constructive requirements,following measures are preferably used, individually or in acombination.

According to an advantageous additional feature, it can be provided thatthe image sensor and the light source are arranged on different sides ofthe partially transmitting mirror. Another preferred feature can be thatthe objective, the light source, the partially transmitting mirror andthe image sensor are arranged in a straight row.

An advantageous feature of the invention can be the fact that theadjustment can be performed by viewing the image of the at least onelight source projected through the objective, by eye, in the objectplane. Thus, no additional technical equipment is necessary for taking apicture; the image of the light source projected through the objectivecan be viewed directly by a user in the object plane and can serve foradjustment purposes.

According to an additional preferred feature, it is proposed that the atleast one light source has a small light emitting surface. The smallerthe light emitting surface, the more exactly the sharpness or theposition of its image point can be recognized on the object. A possiblysmall extension of the light source, being almost point-shaped in anideal case, has the advantage that it becomes easier, in the individualcase of application, to arrange it in a way that the optical path iskept free. Furthermore, it is advantageous if the light emitted from thelight source is as bright as possible, i.e. the light source has a highlight density, as this improves the recognizability of the projectedimage of the light source.

In some cases of application, it can also be advantageous if the lightsource is shaped in a way that it emits light in a line. By means of astraight-line or bow-shaped light source, an advantageous focusingand/or display of the visual field border can be achieved in many casesof application.

In the framework of the invention, light-emitting diodes, in particularthose with a small light surface, are advantageous; these arecommercially available in multiple forms. Light sources which do notcomprise optical elements, e.g. a lens, have proven particularlyadvantageous, as any optical element of the light source would changethe optical path, making it necessary to consider this influence in theconstructive design. In this respect, raw LED chips without housing, orLED's with a housing with a flat light output orifice, are particularlyadvantageous.

One light source is principally sufficient for focusing the opticalimage. This is also true for the adjustment of the image field position,if an appropriate reference point is available on the object. In othercases, however, it can be advantageous to use two or more light sourcesin order to perform the adjustment of additional degrees of freedom. If,for example, it is necessary to align the image sensor in parallel tothe object plane, it can be advantageous to use at least three lightsources, preferably not arranged in a collinear way, as the position ofa plane is defined by three points. In this case, the light sources arepreferably arranged in a plane parallel to the light-sensitive surfaceof the image sensor.

A preferred feature, in particular for adjusting the image field, can bethat the at least one light source is arranged in relation to the imagesensor, in a way that its image is projected by the partiallytransmitting mirror and the objective, onto a position of the objectwhich is in the area of the border of the visual field of the imagesensor. Thereby, it is possible to visualize directly the border of theobject range imaged onto the image sensor, and thus the visual field orimage field, respectively, when the image of the light source isprojected onto the object. According to the embodiment of the camera andthe prevailing practical circumstances, the visual field is given by anaperture located at the outside of the camera, the aperture of thecamera, or by the extension of the image sensor itself.

BRIEF DESCRIPTION THE DRAWINGS

The invention will subsequently be explained in detail with the help ofan exemplary embodiment shown in the figures. The features describedthere can be applied single or in a combination in order to createpreferred embodiments of this invention. In the figures:

FIG. 1 shows a principal sketch of an optical path and a designaccording to the invention,

FIG. 2 shows an arrangement of four light sources on a support,

FIG. 3 shows the projected image of the light sources according to FIG.2 on the object,

FIG. 4 shows a modified support for the light sources,

FIG. 5 shows the support according to FIG. 4 with light sources and

FIG. 6 a longitudinal section through a camera according to theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the optical path and the design according to the invention.It comprises a camera 1 with an image sensor 2 and an objective 3 forimaging an object 4 onto the image sensor 2. The object 4 is located inthe object plane 5, which can be, during the adjustment, e.g. a whitepiece of paper. The optic axis 6 is also shown in the optical path.

The image sensor 2 is, e.g. a unidimensional (line) or bidimensional(matrix) CCD sensor, a CMOS sensor or a photo diode array. All kind ofimage sensors, i.e. semiconductor sensors, in form of CCD line or CCDmatrix, CMOS line or CMOS matrix, photo diode lines or photo diodearrays, camera tubes, CID lines or CID matrix, vidicons, newicons,plumbicons etc., as well as spread individual detectors, i.e. photodiodes, or positions-sensitive detectors, e.g. PSDs, can be consideredhere. For example, a Hewlett Packard/Agilent HDSC 2000 CMOS sensor canbe used.

For the objective 3, arbitrary optical systems can be used, fromindividual, simple, uncorrected lenses up to complex, correctedobjectives. The use of zoom objectives is possible, too. For example, aSK12020S objective (Sekinos/Korea), 12 mm focal length, can be used.

The special feature in FIG. 1 consists in the light sources 7 and thepartially transmitting mirror 8, serving for the adjustment of thedistance between the camera 1 and the object 4, or the object plane 5,respectively, or for the adjustment of the objective 3, or for theadjustment of the position of the camera with respect to the object 4,in order to adjust the image field.

The partially transmitting mirror 8 is located between the objective 3and the image sensor 2, arranged in parallel to the light-sensitivesurface of the image sensor 2. The degree of partial reflectivity can beadapted to the practical experience, it can be between 1% and 99%, inpractice, values around 50% have proven appropriate. For example, apartially transmitting mirror of the type Melles Griot 03BTF007, can beused. The partially transmitting mirror can serve, depending on therequirements, also as a filter, e.g. an IR rejection filter, an IR passfilter, a color filter or a polarization filter, for example with anadditional coating and/or by the selection of an appropriate glass orplastic material.

The light sources 7 are LEDs, for example of the type Kingbright,Rheinmetall-Elektronik KP-1608SRC/Waterclear, 660 nm of GaAlAs, showinga width of 0.8 mm, a length of 1.6 mm and a height of 1.1 mm. The lightsources 7 are arranged between the objective 3 and the partiallytransmitting mirror 8; they emit light towards the partiallytransmitting mirror 8. They are located on a support 9, for example aprinted circuit board, comprising an aperture 10 for the optical beampath.

The distance a between the light sources 7 and the partiallytransmitting mirror 8 is equal to the distance b between the mirror 8and the image sensor 2. As far as the respective elements extend towardsthe optical axis 6, the previously mentioned distance specificationsrelate to the light-sensitive surface for the image sensor 2, themirrored surface for the partially transmitting mirror 8 and thelight-emitting spot for the light sources 9.

By this special arrangement, it is achieved that the light emitted bythe light sources 7 is reflected by the partially transmitting mirror 8,being projected there through the aperture 10 of the support 9 into theobjective 3, and from there to the object plane 5, wherein the objectplane 5 for the imaging of the object 4 onto the image sensor 2 is, atthe same time, the image plane for imaging the light sources 7 onto theobject plane 5. This results in the fact that the light projected fromthe light sources 7 onto the object plane 5, is focused when the imageof the object 4 taken by the image sensor 2 is focused, too. Therefore,it is possible to check the focusing of the image with a screen, forexample a white piece of paper, and, if necessary, to adjust thedistance between camera 1 and object 4 or to adjust the objective 3. Thedistance between the camera 1 and the object 4, or the adjustment of theobjective 3 for which the light sources 7 are imaged best focussed ontothe object plane 5, is the distance which achieves the best focussedimaging of the object 4 onto the image sensor 2.

Thus, the objective 3 serves, on one hand, for the imaging of the object4 onto the image sensor 2, and on the other hand for imaging the lightemitted by the light sources 7 and reflected by the partiallytransmitting mirror 8 onto the object plane 5. The virtual distancebetween the light sources 7 and the objective 3 is equal to the distancebetween the image sensor 2 and the objective, so that the projectedlight sources 7 always indicate the image sharpness for thecorresponding distance setting of the objective; this is evenindependent from the focal length of the objective 3. Due to theparallel arrangement of the partially transmitting mirror 8 with respectto image sensor 2, the structure can be kept compact, inexpensive andeasily adjustable.

The emitting arrangement of the light sources 7 towards the intersectionpoint 11 of the optical axis 6 of the camera 1, with respect to theimage sensor 2, can be an advantageous feature. By this, the brightnessof the light projected onto the object plane 5 can be increased. if thelight sources 7 show an anisotropic emission characteristic.

If the shown device is to serve also for the adjustment of the imagefield of the image sensor 2, the light sources are preferably arranged,with respect to the image sensor 2, in a way that their image isprojected by the partially transmitting mirror 8 and the objective 3onto a place in the object plane 5 or on the object 4, respectively,which is located in the border range of the visual field of the imagesensor 2. Thereby, the light sources 7 can be arranged in a way thattheir images are located in the object plane 5 around the outline of theobject 4 to be imaged, i.e. the image field of the projected lightsources 7 corresponds to the visual field of the image sensor 2, so thatthe visual field of the image sensor 2 can be seen directly by viewingthe position of the image points of the light sources 7 in the objectplane 5, i.e. on an object or on a screen.

FIGS. 2 and 3 show this. FIG. 2 shows a view of a support 9, for examplea printed circuit board. It comprises an aperture 10 for the opticalpath, and the light sources are arranged directly at the border, or atthe periphery, respectively, of this aperture 10. Preferably, the lightsources 7 are arranged in a way that their position corresponds to theimage section to be imaged, thus, in the example shown, in the cornersof the rectangle of the aperture 10, limiting the image field.

In FIG. 3, the image—if necessary, magnified—of the light sources 7 inthe object plane 5, is shown. The area on the object 4 corresponding tothe aperture 10, imaged on the image sensor 2, thus the visual field 13of the image sensor 4, is indicated by a shaded rectangle. In practice,this rectangle is of course not visible, but its position can berecognized due to the location of the image 12 of the light sources 7.Thereby it is possible to adjust the camera 1 by shifting it, in a waythat the desired range of the object 4 is imaged on the image sensor 2.

It is clear that also a smaller or bigger amount of light sources 7 canbe used; these can also be used in another arrangement, e.g. along theborders of the visual field 13, or arranged as light sources 7 emittingin lines. In case of a rectangular visual field 13 of an image sensor 2,preferably four light sources 7 are provided, the images of which are inthe object plane 5 in the corner points of the visual field 13.

If, for example, the light-sensitive surface of an image sensor 2 is arectangle of 4.8 mm×3.6 mm, and the position of the object points imagedon this rectangle is to be displayed in the object plane 5, preferably 4light emitting diodes should be arranged on the support 9 in a way thattheir light emitting elements are positioned as exactly as possible atthe corners of a rectangle of 4.8 mm×3.6 mm. The aperture 10 for theoptical path on the support 9 is only insignificantly smaller than therectangle of 4.8 mm×3.6 mm, depending on the selected distances of theindividual light sources and the focal length of the used objective 3.

FIG. 4 shows a schematic top view of a prepared support 9 with aperture10, and in FIG. 5, this support according to FIG. 4 is shown withmounted light sources 7 and mounting bores ready for mounting into atube-shaped casing of a camera. The length L of the aperture 10 isapproximately 4.8 mm, the width B approximately 3.6 mm and the diameterD of the support 9 is approximately 25 mm. In order to keep theshadowing of the light as low as possible, the light sources 7 in FIG. 5are arranged with an angle of 45° with respect to the longitudinal andtransversal borders of the aperture 10. This allows to provide theaperture 10 in the support 9 with a curvature radius r in the corners,which may be necessary due to manufacturing processes. A certain“overhang” of the light emitting diodes can also be provided, for whichthe connections of the light emitting diodes on the support, theso-called pads and lands, are completely pronounced, but not theclearance between the pads. These measures allow to position the lightsources as close as possible to the corners of the visual field, thusprojecting an image of the visual field into the object plane 5 which isas exact as possible.

FIG. 6 shows a longitudinal section through a section of a camera 1equipped with a device according to the invention. It comprises atube-shaped housing 14 with a support 9 inserted according to FIG. 5.The distance a between the light sources 7 and the mirroring layer ofthe partially transmitting mirror 8 should correspond as exactly aspossible to the distance b between the mirroring layer of the partiallytransmitting mirror 8 and the light-sensitive surface of the imagesensor 2. This can be ensured by appropriate constructive measures or byadjusting elements. For example, punched plastic packing shims, variablein thickness, can be used, also allowing a combination in order toobtain the correct distance.

LIST OF REFERENCE SIGNS

-   1 Camera-   2 Image sensor-   3 Objective-   4 Object-   5 Object plane-   6 Optical axis-   7 Light source-   8 Partially transmitting mirror-   9 Support-   10 Aperture-   11 Intersection point 2-6-   12 Image to 7-   13 Visual field-   14 Housing-   a Distance 7-8-   b Distance 8-2-   L Length-   B Width-   D Diameter-   r Curvature radius

1. Camera with an image sensor for taking a picture, with an objective for imaging an object on the image sensor and with a device for adjusting the distance between the camera and the object, or for adjusting the objective, comprising a partially transmitting mirror, arranged between the objective and the image sensor, and at least one light source, wherein the partially transmitting mirror is oriented in parallel to the light-sensitive surface of the image sensor, the at least one light source is arranged between the objective and the partially transmitting mirror and emits light towards the partially transmitting mirror, and the distance between the light source and the mirror is as long as the distance between the mirror and the image sensor.
 2. Camera according to claim 1, wherein the image sensor and the light source are arranged on different sides of the partially trmsmitting mirror.
 3. Camera according to claim 1, wherein the objective, the light source, the partially transmitting mirror and the image sensor are arranged in a straight row.
 4. Camera according to claim 1, wherein the adjustment can be performed by viewing the image of the at least one light source projected through the objective, by eye, in the object plane.
 5. Camera according to claim 1, wherein the at least one light source has a small light emitting surface.
 6. Camera according to claim 1, wherein the at least one light source is shaped in a way that it emits light in a line.
 7. Camera according to claim 1, wherein the at least one light source is a light emitting diode.
 8. Camera according to claim 1, wherein the at least one light source does not comprise an optical element, e.g. a lens.
 9. Camera according to claim 1, wherein it comprises at least three light sources.
 10. Camera according to claim 9, wherein the light sources are arranged in a plane parallel to the light-sensitive surface of the image sensor.
 11. Camera according to claim 1, wherein the at least one light source is arranged in relation to the image sensor, in a way that its image is projected by the partially transmitting mirror and the objective, onto a position of the object which is in the area of the border of the visual field of the image sensor.
 12. Camera according to claim 11, wherein it comprises various light sources arranged in a way that their images are located in the object plane along the outline of the object to be imaged.
 13. Camera according to claim 12, wherein it comprises four light sources the images of which are located in the object plane in the corner points of a rectangular visual field of the image sensor.
 14. Camera according to claim 1, wherein the at least one light source is arranged in a way that it emits towards the intersection point of the optical axis of the camera with respect to the image sensor.
 15. Camera according to claim 1, wherein the at least one light source is arranged on a support comprising an aperture for the optical path, wherein the at least one light source is arranged in the area of the border of the aperture.
 16. Camera according to claim 1, wherein the objective is a zoom objective.
 17. Method for adjusting the distance between a camera and an object or for adjusting the objective of the camera, wherein the camera comprises an image sensor for taking a picture and an objective for imaging an object onto the image sensor, wherein the camera is equipped with a partially transmitting mirror, arranged between the objective and the image sensor, and at least one light source is provided, wherein the partially transmitting mirror is oriented in parallel to the light-sensitive surface of the image sensor, the at least one light source is arranged between the objective and the partially transmitting mirror and emits light towards the partially transmitting mirror, wherein the distance between the light source and the mirror is as long as the distance between the mirror and the image sensor, and the adjustment of the distance between the camera and the object, or the adjustment of the objective is performed according to the image sharpness of the image of the at least one light source in the object plane.
 18. Method according to claim 17, wherein the image sensor and the light source are arranged on different sides of the partially transmitting mirror.
 19. Method according to claim 17, wherein the objective, the light source, the partially transmitting mirror and the image sensor are arranged in a straight row.
 20. Method according to claim 17, wherein the adjustment can be performed by viewing the image of the at least one light source projected through the objective, by eye, in the object plane.
 21. Method for adjusting the position of a camera in relation to an object, wherein the camera comprises an image sensor for taking a picture and an objective for imaging an object onto the image sensor, wherein the camera is equipped with a partially transmitting mirror, arranged between the objective and the image sensor, and at least one light source is provided, wherein the partially transmitting mirror is oriented in parallel to the light-sensitive surface of the image sensor, the at least one light source is arranged between the objective and the partially transmitting mirror and emits light towards the partially transmitting mirror, wherein the distance between the light source and the mirror is as long as the distance between the mirror and the image sensor, and the at least one light source is arranged in relation to the image sensor, in a way that its image is projected by the partially transmitting mirror and the objective, onto a position of the object which is in the area of the border of the visual field of the image sensor, and the position of the camera in relation to the object is adjusted according to the position of the image of the at least one light source projected onto the object.
 22. Method according to claim 21, wherein various light sources are used, arranged in a way that their images are located in the object plane along the outline of the object to be imaged.
 23. Method according to claim 22, wherein four light sources are used, the images of which are located in the object plane in the corner points of a rectangular visual field of the image sensor.
 24. Method according claim 21, wherein the image sensor and the light source are arranged on different sides of the partially transmitting mirror.
 25. Method according claim 21, wherein the objective, the light source, the partially transmitting mirror and the image sensor are arranged in a straight row.
 26. Method according to claim 21, wherein the adjustment can be performed by viewing the image of the at least one light source projected through the objective, by eye, in the object plane. 